The present disclosure relates to digital printing systems having plural tandem marking or printing engines of the type with seamed endless photoreceptor belts. In such printing systems, it is common practice to invert the sheet after marking on one side thereof in a first of the printing engines and for feeding the inverted sheet into a second printing engine for marking on the opposite side of the sheet to thus facilitate high speed duplex digital printing. However, in printing systems of this type arrangement, problems have been encountered in proper registration of the leading edge of the inverted sheet onto the photoreceptor of the second printing engine for proper placement of the image on the sheet and for avoiding the seam in the photoreceptor of the second marking engine. Where the inverted sheet from the first marking engine is transported by a transporter to the second marking engine, errors in timing, transport speed and positioning of the sheet can accumulate to cause misregistration of the sheet on the second photoreceptor. This is particularly troublesome in view of the requirement that the sheet be placed on the second photoreceptor within a window of plus or minus 30 milliseconds timing with respect to the movement of the photoreceptor.
Typically, tandem marking engines employed for duplex printing operate to synchronize the position of the seams by varying the speed of the photoreceptor in the second marking engine and can result in problems with front to back image-to-paper registration due to paper shrinkage from heating in the first marking engine's fuser and differences in the photoreceptor belt length causing varied photoreceptor speed.
Heretofore digital printing systems employing tandem marking engines for duplex printing have operated in accordance with the procedure shown in FIG. 3 wherein at step 60 the system schedules the arrival times of the sheet stock in the initial and subsequent marking engines; and, proceeds to have the feeder eject the sheet stock at step 62 to meet the scheduled arrival time as determined in step 60, at step 64 arrives at the entrance of the first marking engine and is registered thereon at step 66 for upper registration for marking. At step 68, the sheet is registered for image transfer from the photoreceptor belt and arrives at the discharge exit at the first marking engine at step 70. The system then submits the sheet stock to the inverter at step 72; and, at step 74 the inverter discharges the sheet stock after a fixed dwell time.
Thus, it has been desired to provide a way of improving the registration of the leading edge of sheets emanating from a first tandem marking engine onto the second marking engine.